The present invention relates to a radio communication analyzer and, more specifically, a radio communication analyzer suited for measurement of a plurality of types of communication systems.
Recently, great efforts are being made to develop and put into practical use digital communication systems such as mobile communication systems using cordless and/or cellular telephones.
These communication systems are based on the specifications of various methods such as PDC, PHS, GSM, and CT-2. In these methods, digital data transmission is performed.
A radio communication analyzer obtained by modifying a conventional analog communication analyzer is used to test the transmission/reception characteristics of a radio device used in such a digital communication system.
Assume that each communication system operates in a digital form in testing the transmission/reception characteristics of a radio device by using this radio communication analyzer. In this case, since the respective methods use different modulation methods, different bit rates, and different data formats, it is difficult to cope with these communication method by only minor alterations of the conventional analog communication analyzer.
In order to use such a radio communication analyzer for measurement/test processes and the like in digital communication systems, a plurality of radio communication analyzers having dedicated hardware for the respective communication methods such as PDC and PHS may be prepared.
According to another method, digital circuits are replaced in the adaptor or board form in the analog communication analyzer for the respective communication methods, thereby coping with various communication systems.
A digital radio communication analyzer has a plurality of functions to perform various types of measurements on a digital communication system, and hence may be constituted by a plurality of blocks (modules).
In general, the respective modules independently execute various processes to ensure a high processing speed, and software such as programs and reference data must be installed to perform the processes.
Conventionally, such software is generally supplied using a ROM to allow the modules to quickly operate.
As described above, communication systems such as PDC and PHS are already in service domestically, and many different types of communication systems, including communication systems in foreign countries, are actually being used. Under the circumstances, demands have arisen for a radio communication analyzer capable of easily executing measurement of a plurality of types of digital communication systems alone.
Conventionally, however, the hardware must be replaced in accordance with each communication system. It is therefore difficult to easily and simply change the analyzer system for each type of digital communication system.
In various measuring apparatuses each constituted by a plurality of modules including a radio communication analyzer, ROMs are often used to easily supply software to the respective modules at high speed.
According to this method, however, it is difficult to ensure easy system modification of the measuring apparatus and its expandability.
In addition, as described above, mobile communication systems such as cellular telephone systems and cordless telephone systems use signals modulated by various methods, and also use TDMA (Time Division Multiple Access) to effectively use communication lines.
The frequency of a carrier for carrying a signal used in such a mobile communication system is high as several hundred MHz to several GHz.
In general, a spectrum analyzer is used to accurately measure various frequency components contained in such a signal.
FIG. 18 is a block diagram showing the schematic arrangement of a conventional spectrum analyzer for measuring the frequency characteristics of an RF signal to be measured like the one described above.
First of all, an RF signal a to be measured which is input through an input terminal 201 is adjusted to a predetermined level by an attenuator 202. The resultant signal is input to a frequency converter 203.
The RF signal a input to the frequency converter 203 is mixed with a local oscillation signal b from a local oscillator 205 by a signal mixer 204 to be converted into an IF signal having an intermediate frequency.
After the IF signal is band-limited by a bandpass filter (BPF) 206, it is re-mixed with a local oscillation signal b.sub.1 from a local oscillator 208 by a signal mixer 207 to obtain a final IF signal, which is output from the frequency converter 203.
The oscillation frequency of the frequency converter 203 is swept within a predetermined frequency range by a sweep section controller 209.
As a result, a frequency f.sub.I of an IF signal c output from the frequency converter 203 also changes in synchronism with the sweep operation.
The IF signal c whose frequency is decreased and which is output from the frequency converter 203 is input to an RBW filter 210.
The RBW filter 210 uses a band-pass filter having frequency characteristics like those shown in FIG. 19. The RBW filter 210 removes unnecessary frequency components and selects only a necessary IF signal.
The bandwidth (RBW) at a level 3 dB below the peak level at a center frequency f.sub.C of the frequency characteristics of this band-pass filter, i.e., the RBW filter 210, represents the frequency resolution of this spectrum analyzer.
In addition, since the frequency f.sub.I of the IF signal c output from the frequency converter 203 changes in synchronism with the sweep operation, an output signal output from the RBW filter 210 with the lapse of time within one sweep period (sweep cycle) is the time series waveform of each frequency component of the signal to be tested which is converted into the IF signal c upon sweep reception.
The output signal from the RBW filter 210 is first gain-adjusted in an amplifier 211, and then logarithmically converted in a LOG converter 212.
The output signal whose signal level has been converted in dB by the LOG converter 212 is detected by a detector 213.
As a result, the signal detected within the sweep period indicates the size of the time series waveform of the sweep frequency.
If, therefore, the abscissa represents the frequency; and the ordinate, the amplitude, a frequency spectrum waveform is obtained.
The signal representing the frequency spectrum waveform and output from the detector 213 is input to a VBW filter 214.
This VBW filter 214 is constituted by a low-pass filter (LPF) for removing high-frequency components (noise components) from a frequency spectrum waveform 218 finally displayed on a display unit 217 attached to a front panel 219 of a spectrum analyzer like the one shown in FIG. 20.
A peak detector 215 detects the peak value of the analog frequency spectrum waveform output from the VBW filter 214 at each position on the time axis, thereby obtaining a final frequency spectrum waveform 218 having undergone envelope detection.
The signal indicating the final frequency spectrum waveform is converted into digital data by an A/D converter 216.
The frequency spectrum waveform converted into the digital data is displayed on the display unit 217 of the front panel 219.
As shown in FIG. 20, the frequency spectrum waveform 218 of the signal a is displayed on the display unit 217 of the front panel 219.
The frequency spectrum in a wide frequency range and an arbitrary frequency range can be measured changing the sweep frequency range and the frequency display range on the display unit 217.
In addition, by changing the bandwidth (RBW) of the RBW filter 210, the frequency resolution of the spectrum analyzer can be changed to an arbitrary value.
The following problems to be solved are also posed in the spectrum analyzer shown in FIG. 18.
The bandwidth (RBW) of the RBW filter 210 which indicates the frequency resolution of the spectrum analyzer and the passband center frequency f.sub.C must be adjusted, and calibration processes must be performed with respect to the linearity of logarithmic conversion in the LOG converter 212, amplitude deviations caused by switching of the bandwidth (RBW) of the RBW filter 210, and tuning deviations with respect to the sweep frequency range of the bandwidth (RBW).
Especially in the RBW filter 210 of the conventional spectrum analyzer, steep characteristics are obtained by cascade connection of a plurality of filters, and hence higher precision is required.
As described above, in the spectrum analyzer for executing frequency analysis processing for an RF signal by using analog electronic parts, cumbersome calibration must be performed at the start of actual measurement. For this reason, the measurement efficiency may greatly deteriorate.
Even if adjustment and calibration are completed before the start of measurement, the characteristics of the respective parts may vary in accordance with changes in measurement environment. High measurement precision may not therefore be obtained.